Elevators as smart queues

One of things I like about operations is that it shows up in interesting ways in places that you would expect. Take, for example, elevators. One could think of this as a queuing system. Even if we just focus on morning at an office tower when all the action is getting from the ground floor to their offices, it is a nontrivial problem. At a glance, it seems like a pretty simple queuing system in which a person has to wait for a server (i.e., an elevator car) to be free to take him or her to the right floor. But there are some complications. First, this ain’t Bruce Wayne zipping to the Bat Cave. There is room for several people in an elevator car and it would seem efficient to take more than one at a time. Second, there are externalities. If you stop at a floor before mine, you slow me down.

Inspired by the recent opening of the Burj Khalifa in Dubai, NPR had a story about modern elevator management (Inside The Genius, But Asocial Elevator’s Brain, Jan 11). One point made in the article is that there has been relatively modest changes in the hardware of elevators over the last several decades. Instead, the action has been in optimizing the logic that runs elevator systems. The current state of the art is something called Destination Dispatch:

The new Baltimore headquarters of the asset management firm Legg Mason has a similar elevator system. In the lobby, employees scan in their employee ID cards at a turnstile, and an LCD screen flashes which elevator to take. The system already knows where people are going based on their ID cards and generally by the time employees arrive at the elevators, one is waiting.The elevator stops at only one floor.

In many Destination Dispatch elevators, there are no buttons inside. If you accidentally get into another person’s elevator or input the wrong floor, you must wait to exit the elevator to choose another floor.

Supposedly, this is the most energy-efficient way to operate an elevator system. I wonder where the gains come from. The article suggests that under destination dispatch people end up with lonely asocial elevator rides. That cannot be completely true. There are economies of scale in batching customers together. Giving those up has got to be costly. My intuition is that the gains from smart batching, grouping customers to minimize stops on the way to the top.